1. Field of the Invention
The present invention is directed to an analog-to-digital converter circuit and is also directed to a control device for a gradient amplifier of a magnetic resonance tomography apparatus. The analog-to-digital converter circuit can be utilized for all applications wherein high precision, resolution and constancy over time are required. In particular, the analog-to-digital converter circuit is provided for the digitization of an actual current value signal in a control circuit of a gradient amplifier.
2. Description of the Prior Art
In a nuclear magnetic resonance tomography apparatus, magnetic field gradients are produced by gradient coils, each of which is connected to a gradient amplifier. During the measuring event, each gradient coil has a current flowing therein that, for example, can assume values up to 300 A with a predetermined current curve shape. The shape of the current curve must be adhered to exactly up to a few mA. A complicated control circuit is required in order to achieve this precision.
In known gradient amplifiers, the control circuit including the drive electronics for a switched output stage (modulator) is implemented as an analog circuit. This, however, limits the functionality. More complex applications, for example, energy balancing between a number of output stages or a more detailed answerback of individual output stage parameters, cannot be economically accomplished in analog technology.
There is therefore a desire to design the control circuit and modulator as fully digitally as possible, for example with a suitably programmed digital signal processor (DSP). However, the problem of the digitization of the analog actual current value signal determined by a current converter with the required precision, sampling rate and stability thereby arises. A corresponding problem occurs if, instead of the actual current value signal, some other analog signal that is to be digitalized with high precision for input into a digital processing means.
A digital-to-analog converter referred to as "Tracking ADC" is described in the book "Halbleiter-Schaltungstechnik" by U. Tietze and Ch. Schenk, Springer Verlag, 10.sup.th Edition 1993, pages 784-785, (FIG. 23.43 and associated description). This known converter circuit has a differential amplifier which compares an analog input signal to an analog converter signal and generates an analog difference signal corresponding to the difference therebetween, a digital-to-analog converter which generates the aforementioned analog converter signal from a digital converter value, and a control unit which generates the aforementioned digital converter value as the output of the overall circuit. However, the conversion speed of this circuit as well as the zero point stability and noise suppression are low. Employment thereof for gradient amplifiers is not suggested.